The invention relates to a deflection unit for colour cathode-ray tubes, which unit is also called a deflector and comprises a pair of horizontal deflection coils and a pair of vertical deflection coils in the form of a saddle whose particular shape allows the convergence errors to be minimized.
A cathode-ray tube designed to generate colour images generally comprises an electron gun emitting three coplanar electron beams, each beam being intended to excite bands of luminescent material of the corresponding colour (red, green or blue) on the tube""s screen.
The electron beams scan the tube""s screen under the influence of the deflection fields created by the horizontal and vertical deflection coils of the deflector which is fixed to the neck of the tube. A ring of ferromagnetic material conventionally surrounds the deflection coils so as to concentrate the deflection fields in the appropriate region.
The three beams generated by the electron gun must always converge on the tube""s screen or else suffer the introduction of a so-called convergence error which, in particular, falsifies the rendition of the colours. In order to achieve convergence of the three coplanar beams, it is known to use so-called self-converging astigmatic deflection fields; in a self-converging deflection coil, the intensity of the horizontal-deflection field then has a pincushion-shaped distribution and that of the vertical deflection field has a barrel-shaped distribution.
Coma is an aberration which affects the side beams coming from an electron gun having three beams in line, independently of the astigmatism of the deflection fields and of the curvature of the screen surface of the tube; these side beams enter the deflection region at a small angle with respect to the axis of the tube and undergo a deflection in addition to that of the axial beam. The coma is generally corrected by modifying the distribution of the deflection fields at the point where the beam enters the deflector so that the coma generated compensates for that produced by the field distribution necessary to obtain the desired astigmatism for self-convergence. Thus, with regard to the horizontal deflection field, the field at the rear of the deflector has the shape of a barrel and in the front part has the shape of a pincushion.
Field configurations like those described above may cause the appearance of errors in the convergence of the side electron beams, which are manifested on a rectangular test pattern displayed on the tube""s screen by a shift in the red image with respect to the blue image along the vertical edge of the test pattern. If the shift varies in a linear manner, it is possible, in a known way, to remedy it; on the other hand, if the value of the red/blue convergence error does not vary in a linear manner along the vertical edge of the test pattern, there was not hitherto a means for remedying this phenomenon, called xe2x80x9cvertical blue wavexe2x80x9d, in a simple manner.
Moreover, the problems of geometry of the image formed on the tube""s screen, of coma and of convergence are associated with the planarity of the screen and with the size of the latter. Conventional cathode-ray tubes manufactured a few years ago and using a screen of spherical shape generally have a small radius of curvature. Since the current trend is moving towards screens of large radius of curvature, or else completely flat screens, with diagonals greater than 70 cm in length, it is becoming increasingly difficult to control the abovementioned problems solely by means of suitable fields generated by the deflection coils.
It is common practice to divide the deflection system into three successive action regions along the main axis of the tube: the rear region closest to the electron gun influences more particularly the coma, the intermediate region acts more particularly on the astigmatism of the deflection field and therefore on the convergence of the red and blue electron beams and, finally, the front region, lying closest to the tube""s screen, acts on the geometry of the image which will be formed on the tube""s screen.
The invention aims to provide a solution to the xe2x80x9cvertical blue wavexe2x80x9d problem without adding additional components such as metal pieces arranged so as to locally modify the fields of the permanent magnet or magnets, which solution provides a means of additional control without moreover impairing the other characteristics of the deflection device, the correction of one characteristic generally causing degradation of one or more others.
To do this, the electromagnetic deflection unit for colour cathode-ray tubes according to the invention comprises a pair of horizontal deflection coils and a pair of vertical deflection coils, the vertical deflection coils being in the shape of a saddle and comprising a front bundle lying on the screen side of the tube and a rear bundle lying on the electron-gun side, the said bundles being connected together by lateral conductor harnesses, the front and rear bundles and the lateral harnesses defining a window free of conductors, which deflection unit is characterized in that, in the region close to the front bundle, the window extends over a radial angular aperture "PHgr" of greater than 38xc2x0.